It is known that the purification of municipal and industrial wastewater is often carried out biologically. In recent decades, processes have passed from those using free microorganism cultures to processes with cultures fixed on specific growth media for the purpose of reducing the size of purification plants.
Fixed cultures are employed either as a fixed bed, that is to say a microorganism growth medium is stationary in the reactor, or as a moving bed, in which case the support materials are small elements that can move freely in the zone of contact with the polluted water. These support elements may be moved either by mechanical stirring or by injecting a liquid, or else by injecting a gas, especially air (this air possibly being, for example, the air needed for the microorganisms to operate when they are aerobic).
The creation and maintenance of a certain level of turbulence in the reaction medium are useable for continuously abrading and cleaning the support material for the microorganisms, this turbulence furthermore making it possible to limit the accumulation of fixed biological sludge. Such turbulence may be created, for example, by the intensity of the gas injected into the medium. Reference may be made in this regard to EP-A-0 549 443.
If it is desired to treat the pollution due to carbon and to nitrogen simultaneously, it is possible to find advantageous solutions given that the materials serve as growth medium for a certain nitrifying biomass, the growth of which is much higher than that in the absence of these materials (see EP-A-0 549 443): these are referred to as hybrid cultures.
However, these known systems have a number of drawbacks. Thus, in the method described above, the production of biological sludge is tied to the normal growth metabolism of the bacteria decontaminating the water. Furthermore, the growth medium materials used are held in place in the reaction chamber either by a retention screen (that lets water through, but not the support material) or by means of a specific separation system. The major drawback of screens is their clogging.